Ok, now I have tried EVERYTHING possible. I have reinstalled Calibre, deleted all files beloning to it (not the bibliothek as it is not in the standard path), installed it in english, restarted the computer after deinstallation, started with a new bibliothek. Nothing really nothing changed the problem. I get always the error message the the file isn't a zip-file. I reloaded it with various browsers and tools, no change.
This plugin isn't the only one and as the version of Calibre advances I am unable to install plugins. So I still can't say it is the plugin or Calibre. I will open a case there too. It just annoying. Serveral plugins just can't be installed with such an idiotic error message as I can easily unzip those files without any problem.
The Goodread Covers and Metadata installs without problems, Search the Internet, Kindle Collections only with version 0.7.48 or earlier, I just can't see where the problem is.
The both Goodread plugins seem to be without function, I can't see something there, Kindle Collections won't run at all as it always states it must be initialized even during initialization.
These are really cool plugins and I really would like to use them but I just have no idea how to do so.